1. Field of the Invention
The present invention relates to an optical device for coupling an optical component such as a plurality of single core fiber, to an optical element such as a multi-core fiber.
2. Related Background of the Invention
Conventionally, a multi-core fiber coupling device for coupling a plurality of single core fibers to a multi-core fiber is known. For example, Japanese unexamined Patent Publication No. 60-212710 discloses a device in which a lens is interposed between a multi-core fiber having two core regions and two single core fibers in order to branch the multi-core fiber. The lens in this device deflects a plurality of beams emitted from the multi-core fiber so as to be separated from each other in a direction inclined relative to an optical axis of the multi-core fiber.
According to the above-mentioned conventional art, since the lens inclines the beams from the multi-core fiber, in accordance with the inclination, the single core fibers need to be inclined. In this case, angular adjustment and alignment between the multi-core fiber and the single core fibers are so difficult, which is impractical.
Therefore, the Inventors have made a study on a device as shown in FIG. 1 as a more practical device. The device shown in FIG. 1 includes a lens L1 (focal length f1) that separates the plurality of beams from a multi-core fiber 10 from each other, and a lens L2 (focal length f2) that deflects the plurality of beams separated from each other by the lens L1 in a direction parallel to the optical axis of the multi-core fiber. For this reason, since there is no need to incline single core fibers 20 relative to the multi-core fiber 10, angular adjustment becomes unnecessary and high practicality can be achieved.
Here, the interval between the plurality of beams from the multi-core fiber 10 is extended by the lens L1, and an interval extending factor m is f2/f1. In contrast, in accordance with the Lagrange's Law used in the optical field, a beam spread angle θ is proportional to a reciprocal of the interval extending factor. That is, in the device shown in FIG. 1, given a beam spread angle θOUT on an end surface of the multi-core fiber, a spread angle (condensing angle) θIN on end surfaces of the single core fibers becomes θOUT/m.
In the case where the beams emitted from the multi-core fiber are Gaussian beams, given that a beam radius on the end surface of the multi-core fiber is wOUT and wavelength is λ, the spread angle θOUT satisfies a following equation.θ=λ/(π/w)
Meanwhile, π is a circular constant. The above-mentioned equation is also applied to beams incident on the single core fibers. The spread angle θIN of the beams incident on the single core fibers becomes θIN/m according to the Lagrange's Law. In this case, a beam radius wIN on the end surfaces of the single core fibers is multiplied by m according to the above-mentioned equation to become m/wOUT. Consequently, there has been a problem in which optical coupling loss to the single core fibers becomes large.
The present invention has been made to solve the above-mentioned problem, and its object is to provide an optical device that reduces coupling loss while improving practicality.